A novel optical method to measure oxygen concentration is proposed based upon the oxygen dependent quenching of the phosphorescence of selected phosphors. The method will be developed for rapid (msec) response to changes in oxygen tension in aqueous solutions and accurate measurement in the range of 10-5 to 10-8 M. Probes will be selected which have suitable lifetimes, are chemically stable and which emit in the green or red spectral region. The advantages of the proposed sensor over existing technology includes stability and sensitivity, especially in the lower range of tissue oxygen concentrations (less than MuM). Several configurations of the instrument will be developed: 1) For measurement of bulk phase oxygen, the probes will be incorporated into oxygen permeable membranes, 2) For measurement of intracellular oxygen, probes will be incorporated into the cell, 3) For measurement of tissue oxygen concentrations, the probes will be incorporated into catheters or needle probes which can be inserted into the tissue. The oxygen sensor will be used to study the oxygen dependence of several important drug metabolizing systems of hepatocytes. These include the microsomal reactions of N-demethylation of ethylmorphine and O-demethylation of p-nitroanisole, peroxisomal oxidation of fatty acids and D-amino acids, and two reactions involved in synthesis and degradation of catacholamines: tyrosine hydroxylase and monoamine oxidase.